In recent years, liquid crystal display devices have become common, in place of cathode-ray tube devices. Merits of the liquid crystal display devices in terms of low energy consumption, thinness, lightness and the like have indeed enabled them to be used in a wide variety of applications, such as liquid crystal display televisions, monitors, and portable telephones. The improvement of an illumination device (also called a backlight) provided behind a liquid crystal display device has been suggested as a way to further enhance the above merits.
The illumination devices are generally divided in two main categories: side light type illumination devices (also called edge light type illumination devices) and direct type illumination devices. A side light type illumination device is configured so that a light guide body is provided behind a liquid crystal display panel and a light source is provided at an end portion of the light guide body. Light emitted from the light source is reflected by the light guide body, and is indirectly and evenly irradiated onto the liquid crystal display panel. With this configuration, it is possible to achieve an illumination device having both a thin-structure and excellent luminance uniformity, although its luminance is low. For this reason, the side light type illumination devices are mainly used in small- and medium-sized liquid crystal displays, such as portable telephones and laptop computers.
An example of the side light type illumination device is proposed in Patent Literature 1. Patent Literature 1 mentions a surface light-emitting device including a light guide body having a reflecting surface on which a plurality of dots are formed so that a light-emitting surface emits light uniformly. In this surface light-emitting device, light does not reach corners of the reflecting surface due to directivity of a light source, which results in the corners of the reflective side becoming dark. In order to avoid this, this surface light-emitting device has the dots provided so that their density becomes higher in the corners of the reflecting surface than in the other parts of the reflecting surface.
On the other hand, a direct type illumination device includes a plurality of light sources disposed behind a liquid crystal display panel so as to directly emit light to the liquid crystal display device. Accordingly, the direct type illumination device can easily achieve a high luminance even with a large screen. The direct-type illumination devices are therefore mainly used in large-size liquid crystal displays having a screen size of 20 inches or over. In this regard, existing direct type illumination devices are as thick as 20 mm to 40 mm. This prevents further reduction in the thickness of the display.
In this view, shortening a distance between the light sources and the liquid crystal display panel would make it possible to achieve an even thinner large liquid crystal display device; however, in such a case, it is necessary to increase the number of light sources in order to obtain luminance uniformity of the illumination device. The increase in the number of light sources, however, results in a cost increase. For this reason, it has been desired to develop a thin illumination device having excellent luminance uniformity, without any increase in the number of light sources.
Conventional technologies have attempted to solve the above problem. An objective of these attempts is to achieve thinner large liquid crystal displays by providing a plurality of side light type illumination devices.
For example, Patent Literature 2 proposes a surface light source device which can be suitably used in a large liquid crystal display, because the surface light source device can secure a wide light-emitting area with a compact structure. This surface light source device includes (i) plate-like light guide blocks arranged in a tandem manner and (ii) a primary light source to supply first light to each of the light guide blocks.
Illumination devices constituted by aligning a plurality of light-emitting units each including a combination of a light source and a light guide body, as the one described above, are called tandem-type illumination devices.